1. Field of the Invention
The invention relates to the field of high temperature polymers and their use in sealing and other wear-resistant components.
2. Description of Related Art
Sealing components and other wear resistant materials can be used in very rigorous and demanding environments. Their wear and mechanical properties are very critical to their applicability and useful life. For example, sealing components are typically formed of elastomeric materials that are situated in a gland. In one application, an annular seal may fit within a gland and be installed to seal a gap between surfaces, e.g., a seal may be installed around a shaft that fits within a bore and the bore can be configured to have a gland for receiving the seal. In many instances, the seal is not installed alone and is part of a seal assembly. Such assemblies may include back-up rings and other components. Seals and seal assemblies are usually constructed to support the primary sealing element, generally formed of an elastomeric material, to prevent extrusion of that material into the gland and into the space or gap between the sealing surfaces.
When temperatures of use become high, pure elastomeric seals may not be able to provide sufficient sealing force to prevent leakage and/or may extrude into the gap between sealing surfaces, e.g., a shaft and a seal. Under such conditions, thermoplastic materials with higher shear strengths may be used to isolate the soft elastomer component from the gap between the sealing surfaces to assist in resisting extrusion. Combination of harder and softer materials are sometimes also used so that softer materials (such as, for example, polytetrafluoroethylene (PTFE) or other fluoropolymeric materials) are prevented from extruding into the gap by stiffer thermoplastic antiextrusion components. Such materials are used in unidirectional and bidirectional sealing assemblies.
Materials that have been used as antiextrusion components include polyetherether ketone (PEEK) and similar polyketones. Continuous use temperatures for such materials range from about 240° C. to about 260° C., including for commercial polyarylketones, such as Victrex® polyarylenes.
In use, at elevated temperatures, polyketones are well above their glass transition temperatures (Tg). For example, PEEK is semicrystalline and has a Tg of 143° C. Other polyketones such as Victrex® PEK and PEKEKK have respective glass transition temperatures of 152° C. and 162° C.
As semicrystalline materials are used above their glass transition temperatures, they tend to demonstrate lower mechanical properties in service and there is a corresponding drop in performance. With reference to FIGS. 1 and 2, this effect can be seen as PEEK rings are loaded below and above their glass transition temperatures, respectively, and significant differences in extrusion resistance can be seen. FIG. 2 shows a 60% increase in extrusion at a pressure that is 50% lower for the same loading period.
Such extrusion issues are also problematic in the area of electrical connectors. Such connectors are used to relay electrical signals from sensors to electronics in downhole oil exploration tools. They function also as bulkhead seals and are the last line of defense against destruction of electronics in an oil exploration tool when the tool suffers a catastrophic failure. Such seals must be able to withstand high pressure for extended periods of time at elevated temperature. Unfortunately, many downhole oilfield products are used at or above the Tg of various commercial polyketones, so that severe extrusion can take place. Often such extrusion results in failure of the part as a seal, allowing either moisture to leak through the seal or for the part to deform so it no longer performs properly mechanically. An example of this behavior can be seen in FIG. 3, which demonstrates extrusion on an electrical connector.
Attempts to enhance the properties of PEEK have been attempted. Cross-linking has been widely recognized as one way to modify high temperature polymeric materials. Several inventions have been aimed at improving the high temperature performance of organic polymers by using cross-linking within the polymers by cross-linking to itself, grafting cross-linking compounds to the polymer, or by incorporating cross-linking compounds into the polymer such as by blending.
U.S. Pat. No. 5,173,542 discloses use of bistriazene compounds for cross-linking polyimides, polyarylene ketones, polyarylether sulfones, polyquinolines, polyquinoxalines, and non-aromatic fluoropolymers. The resulting cross-linked polymers are useful as interlayer insulators in multilayer integrated circuits. The patent discusses difficulties in the art encountered includes controlling the cross-linking process in aromatic polymers to enhance properties. It proposes a bistriazene cross-linking structure and method to enhance chemical resistance and reduce crazing so that useful interlayer materials may be formed.
Other attempts to cross-link polymers to enhance high temperature properties have encountered difficulty with respect to thermal stability of the polymer. Other issues arise in terms of control of the rate and extent of cross-linking.
U.S. Pat. No. 5,874,516, which is assigned to the Applicant of the present application and is incorporated herein by reference in relevant part, shows polyarylene ether polymers that are thermally stable, have low dielectric constants, low moisture absorption and low moisture outgassing. The polymers further have a structure that may cross-link to itself or can be cross-linked using a cross-linking agent.
U.S. Pat. No. 6,060,170, which is assigned to the Applicant of the present application and is incorporated herein by reference in relevant part, describes the use of polyarylene ether polymer compositions having aromatic groups grafted on the polyarylene ether polymer backbone. The grafts allow for crosslinking of the polymers in a temperature range of about 200° C. to about 450° C. This patent discloses dissolving the polymer in an appropriate solvent for grafting the cross-linking group(s). Such required process steps can sometimes make grafting difficult or not practical in certain types of polymers or in certain polymeric structures, including, e.g., PEEK.
A further patent, U.S. Pat. No. 5,658,994 discusses a polyarylene ether polymer in which the polymer may be crosslinked, e.g., by crosslinking itself through exposure to temperatures of greater than about 350° C. or by use of a crosslinking agent. The patent also describes end-capping the polymer using known end-capping agents, such as phenylethynyl, benzocyclobutene, ethynyl, and nitrile. Limited crosslinking is present at the end of the chain such that relevant properties, i.e., the glass transition temperature, the chemical resistance and the mechanical properties, are not enhanced sufficiently for all high temperature applications,
Further developments in improving polyarylene ether polymer properties are described in International Patent Publication No. WO 2010/019488, which describes use of per(phenylethynyl)arenes as additives for polyarylene ethers, polyimides, polyureas, polyurethanes and polysulfones. The application discusses formation of a semi-interpenetrating polymer network between two polymers to improve properties.
Previous attempts have also been made to control where crosslinks form along high glass transition polymers to garner desired mechanical properties and prepare useful high temperature polymers. U.S. Pat. No. 5,658,994, noted above, and incorporated herein by reference in relevant part, demonstrates the use of a polyarylene ether in low dielectric interlayers which may be cross-linked, in one instance, by cross-linking the polymer to itself, through exposure to temperatures of greater than about 350° C. or alternatively by using a crosslinking agent. In that patent, as well as in U.S. Pat. No. 5,874,516, cross-linking occurs at the ends of the polymer backbone using known end capping agents, such as phenylethynyl, benzocyclobutene, ethynyl and nitrile. There is still a need to control the rate and extent of cross-linking and the location of crosslinks.
Co-pending International Application No. PCT/US2011/061413 describes a composition having a crosslinking compound of the structure:
wherein R is OH, NH2, halide, ester, amine, ether or amide, and x is 2-6 and A is an arene moiety having a molecular weight of less than about 10,000. When reacted with an aromatic polymer, such as a polyarylene ketone, it forms a thermally stable, cross-linked polymer. This technology allows for crosslinking of polymers previously believed non-crosslinkable, and which are thermally stable up to temperatures greater than 260° C. and even greater than 400° C. or more, depending on the polymer so modified, i.e., polysulfones, polyimides, polyamides, polyetherketones and other polyarylene ketones, polyureas, polyurethanes, polyphthalamides, polyamide-imides, aramids, and polybenzimidazoles.
Co-Pending U.S. Provisional Patent Application No. 61/716,800, co-owned by the Applicant of the present application describes a cross-linking composition comprising a cross-linking compound and a cross-linking reaction additive selected from an organic acid and/or an acetate compound. The cross-linking compound has a structure according to formula (I):
wherein A is an arene moiety having a molecular weight of less than 10,000 g/mol, R1 is selected from a group consisting of hydroxide (—OH), amine (—NH2), halide, ether, ester, or amide, and x=2.0 to 6.0, wherein the cross-linking reaction additive is capable of reacting with the cross-linking compound to form a reactive intermediate in the form of an oligomer, which reactive intermediate oligomer is capable of cross-linking an organic polymer.
In one embodiment, the cross-linking reaction additive is an organic acid which may be glacial acetic acid, formic acid, and/or benzoic acid. In another embodiment, the cross-linking reaction additive may be an acetate compound that a structure according to formula (III):
wherein M is a Group I or a Group II metal; and R2 is an alkyl, aryl, or aralkyl group, wherein the alkyl group is a hydrocarbon group of 1 to about 15 carbon atoms having 0 to about 5 ester or ether groups along or in the chain of the hydrocarbon group, wherein R2 may have 0 to about 5 functional groups that may be one or more of sulfate, phosphate, hydroxyl, carbonyl, ester, halide, mercapto or potassium. The acetate compound may be lithium acetate hydrate, sodium acetate and/or potassium acetate, and salts and derivatives thereof. These cross-linking compositions allow for control of a cross-linking reaction when combined with an organic polymer and can enable a lower rate of thermal cure, giving a broader window and better control during heat mold of the resultant cross-linked organic polymer. Such control can enable formation of polymers that are suitable for extreme conditions such as down-hole end applications.
While polyimides and polyamide-imide copolymers have higher glass transition temperatures of about 260° C. or more, they tend to not be useful in strong acids, bases or aqueous environments, as they suffer more easily from chemical attack. As a result, while their operating temperatures are more attractive, their chemical resistance properties limit their usefulness in sealing applications where the fluid medium is water based or otherwise harmful to the material. For example, testing of polyimide by applicant has shown about an 80% loss in properties after aging at 200° C. for three days in steam, using ASTM-D790 to test the flexural modulus.
Fully aromatic polysulfones such as polyether sulfone (PES) and polyphenyl sulfone (PPSU) may be used in such end applications, but their amorphous nature creates issues in that they are vulnerable to stress cracking in the presence of strong acids and bases. Due to the possibility of the amorphous polymers flowing at temperatures near their glass transition temperature over time, continuous use temperatures are typically set about 30° C. to 40° C. below the glass transition temperature. Thus, for continuous use for a polysulfone (PSU), the temperature is recommended to be set at 180° C. when the glass transition temperature is about 220° C.
Other problems encountered in more demanding end uses exposed to harsh chemicals, water and/or steam, include problems associated with a plasticizer effect caused when the polymer absorbs the chemical which can enhance motion of molecular chains and create a depression of the glass transition temperature from its normal state in the unswollen polymer.
A further issue is associated with creep. When polymers operate above their glass transition temperature, creep is a limiting factor for seal components which can deform under harsh conditions. Thus, to improve mechanical properties, prevent creep and resist extrusion, most high temperature polymers in use are filled for use as backup rings or molded components. The downside of use of fillers is that it typically drops the ductility tremendously. For example, unfilled PEEK has a tensile elongation of about 40%, whereas 30% carbon-filled PEEK has a tensile elongation at break of only 1.7%. Thus the material becomes more brittle from the strengthening filler, and the brittleness can result in part cracking under prolonged loadings. The use of fillers also causes a differential coefficient of thermal expansion in the mold versus the transverse direction of the molded parts. This can also cause significant molded-in stress. The end result is cracking over time due to creep rupture, even when a part is not under a significant load.
Thus, there is a need in the art for better and higher performing polymeric materials for sealing components, seal connectors and similar parts that can operate at high service temperatures associated with oilfield and other harsh conditions and industrial uses, but still maintain good mechanical performance, resist extrusion of the seal or connector material into a gap between two surfaces to be sealed or along the pin, and resist creep when in use, without becoming brittle and significantly losing its ductility.